A vehicle generally comprises a hydraulic brake system for reducing down the speed of the vehicle and/or stopping the vehicle. Manipulating the brake pedal by the manual force of the driver is laborious, thus many vehicles are equipped with boosters in addition to the hydraulic-force transmitting mechanisms. A booster generally uses the vacuum in the inlet pipe of the engine as boost force source to increase the brake force of the hydraulic brake system.
The most common booster is the membrane type vacuum booster, which is mounted between the brake pedal and the primary brake cylinder of the hydraulic brake system and comprises a vacuum chamber and a working chamber which are separated in a housing by a membrane, the vacuum chamber being connected with the inlet pipe of the engine, the working chamber being in communication with the atmosphere via an air valve, and the vacuum chamber and the working chamber are connected with each other via a vacuum valve. In a rest state, the air valve is closed and the vacuum valve is opened so that the pressure in the vacuum chamber and the pressure in the working chamber are maintained as substantially the same. In a brake state, in response to a force applied to the brake pedal by the vehicle driver, the vacuum valve is closed and the air valve is opened, so that air is drawn into the working chamber. As a result, the pressure in the working chamber becomes higher than the pressure in the vacuum chamber. The membrane moves under the unbalance of pressure between the vacuum chamber and the working chamber, by means of which, a vacuum boost force which may be several times larger than the pedal force is applied to the primary brake cylinder. In this condition, the primary brake cylinder is subjected to the actions of both the pedal force and the vacuum boost force, so that the output pressure of the primary brake cylinder is increased, and the pedal force may be reduced.
It can be seen that the vacuum booster generates a vacuum boost force using the vacuum formed in the inlet pipe of the engine. When the engine is turned off, the vacuum of the inlet air will not exist, and no enough vacuum boost force will be able to be generated then.